"She was waiting for the answer."
Translation:Она ждала ответа.
The very bottom of the "Tips and Notes" for the Instrumental Case skill has a bunch of less common "cases", including:
“Waiting” case: not much of a case, but actually the verb “ждать” (to wait) would use Accusative for people and things that can affect their appearance and Genitive for everything else (an event/thing that does not choose when to arrive).
So, basically, this is just a special case.
Она хдала ответа - genitive
Она ждала мальчика - accusative of an animate masculine noun (same as genitive)
Она ждала девочку - accusative, clearly not a genitive
Oddly, Duo accepted Она ждала ответ. Shady_Arc commented in another exercise that vehicles and messages could be either - though messages would be Accusative only if they were definite, about to arrive. I expect that vehicles might be treated in a similar way. So, in "she was waiting for the/an answer", accusative case might indicate that she expected the answer to be coming right away. Perhaps standing in front of someone, tapping her foot...